The invention relates generally to downhole tools. More particularly, the invention relates to shock dampers for jars or other downhole equipment that apply an impact force to a downhole assembly.
In oil and gas well operations, it is frequently necessary to apply an axial blow to a tool or tool string that is positioned downhole. For example, application of axial force to a downhole string may be desirable to dislodge drilling or production equipment that is stuck in a wellbore. Another circumstance involves the retrieval of a tool or string downhole that has been separated from its pipe or tubing string. The separation between the pipe or tubing and the stranded tool—or fish—may be the result of structural failure or a deliberate disconnection initiated from the surface. Another example of creating force in downhole operations is with the use of casing perforation tools.
As an example, jars have been used in petroleum well operations for several decades to enable operators to deliver axial impacts to stuck or stranded tools and strings. Drilling jars are frequently employed when either drilling or production equipment gets stuck in the well bore. The drilling jar is normally placed in the pipe string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via manipulation of the drill string. These impact blows are intended to dislodge the stuck object, thereby enabling continued downhole operations. Fishing jars are inserted into the well bore to retrieve a stranded tool or fish. Fishing jars are provided with a mechanism that is designed to firmly grasp the fish so that the fishing jar and the fish may be lifted together from the well. Many fishing jars are also provided with the capability to deliver axial blows to the fish to facilitate retrieval.
Conventional jars typically include an inner mandrel disposed in an outer housing. The mandrel is permitted to move axially relative to the housing and has a hammer formed thereon, while the housing includes an anvil positioned adjacent to the mandrel hammer. By impacting the anvil with the hammer at a relatively high velocity, a substantial jarring force is imparted to the stuck drill string. If the jarring force is sufficient, the stuck string will be dislodged and freed. However, while the jarring force may be sufficient to dislodge the stuck string, the force may be so large as to damage the remaining components of the downhole tool if too much force is transferred to the other components.